I would like to know if the css code "display:none;" affects SEO negatively because I recently downloaded a wordpress theme and I wanted to remove some stuffs that I don't like to see (such as featured posts etc)

Here are my problem summary:

1.) When I try to edit HTML, after I click update file my changes doesn't apply when I check the website (I already fixed chmod settings)

2.) Should I still use css "display:none;" for the stuffs I don't want to see in the theme I've downloaded? I'm not sure if it will affect SEO negatively

Will really help if anyone can reply to my uber noob questions. I'm struggling here and trying to learn it (I've spent like 2 hours trying to fix chmod and stuff LOL)

I'm sorry if I always post in forum asking for some help, this is the only way I can learn CPA and website SEO. I hope I'm not annoying anyone here.